1. Field of the Invention
The present invention relates to an automatic gain control for a power amplifier. More particularly, the present invention relates to an apparatus and method for automatically controlling a gain of a power amplifier in which a circuit made via a complementary metal oxide semiconductor (CMOS) process is used to control the gain and effectively control a mismatched impedance caused by gain control.
2. Description of the Related Art
Automatic gain control refers to controlling a gain to maintain a magnitude of an output signal at a constant level. The magnitude of the output signal varies according to a communication distance, i.e., a distance between a base station or a repeater and a terminal. For example, for wireless communication, a gain of a power amplifier is controlled using an appropriate transmission power that compensates for interference between other users of mobile stations and for signal attenuation due to a communication distance. Generally, a power amplifier performing a gain control function includes an automatic gain control unit and a power amplifying unit.
In a conventional wireless communication terminal, the current of the power amplifier is reduced such that gain control is performed, thereby causing a nonlinear distortion of an output signal. However, this method of reducing current to control gain reduces dynamic range, thereby deteriorating the performance of the power amplifier. In order to eliminate this disadvantage, a conventional gain control unit additionally includes an off-chip component formed of a compound having good linearity. This external component, however, increases manufacturing cost and circuit size, and complicates circuit integration. Further, the conventional automatic gain control unit consumes additional power, since the gain control is still realized by reducing the current.